COT (constant on-time) control method is widely used in power supply area due to its simple structure and excellent transient response. Nevertheless, for switching converters with COT control, there may be a sub-harmonic oscillation at the output voltage because of the insufficient ESR (equivalent series resistance) of the output capacitor.
To avoid this sub-harmonic oscillation, a compensation signal in phase with the inductor current is often employed. FIG. 1 illustrates a schematic of a prior COT switching converter 100, wherein a ramp compensation generator consisting of a resistor Rramp and a capacitor Cramp is coupled to the inductor L in parallel. The compensation signal RAMP generated by the ramp compensation generator is added to a feedback signal FB to form a sum signal FB1. When the sum signal FB1 becomes smaller than a reference signal Vref, the switch M1 turns on and the switch M2 turns off. When the on-time of the switch M1 reaches a time threshold determined by an on-time control circuit, the switch M1 turns off and the switch M2 turns on.
When designing the compensation signal RAMP, there is always a tradeoff between the stability and transient response. A large compensation signal will help on the stability, but will worsen the transient response and even cause double pulse at light load.
Based on FIG. 1, the amplitude Vramp of the compensation signal RAMP can be expressed as
                              V          ramp                =                                                                              V                  in                                -                                  V                  out                                                                              R                  ramp                                ⁢                                  C                  ramp                                                      ⁢                          t              on                                =                                    (                              1                -                                                      V                    out                                                        V                    in                                                              )                        ⁢                                                            V                  out                                ⁢                                  T                  s                                                                              R                  ramp                                ⁢                                  C                  ramp                                                                                        (        1        )            Wherein Vin represents the input voltage, Vout represents the output voltage, ton represents the on-time of the switch M1 and Ts is the switching period.
In practical applications, the output voltage Vout and the switching period Ts are generally constant. Therefore, it is clear from the equation (1) that the amplitude Vramp will vary along with the input voltage Vin, which means that the compensation signal Vramp shown in FIG. 1 is not suitable for applications with wide input range. The ramp signal RAMP may be too small in low input situations and too large in high input situations.